 |
NSW Arbovirus Surveillance & Vector Monitoring Program |  |
|
NSW Arbovirus Surveillance & Vector Monitoring Program | |
 1999 - 2000
Annual Report  |
The moist
conditions that marked the end of the drought in 1998 continued into 1999, with rainfall
being mostly above average across the state for 1999-2000. Despite this, mosquito numbers
failed to reach extremely high numbers during peak season. This appeared to be due to the
cold summer temperatures, with some areas up to 60C below average. Autumn
however, was quite mild and warm, and mosquito numbers remained stable and above average
until the end of April. Thus, while the overall number of mosquitoes was not as high as
the previous season, the 245,524 mosquitoes collected during 1999-2000 was the second
highest number trapped by the program.
The only
locations that had very high mosquito densities were those from the northwest, where
mosquitoes were extremely abundant following the heavy February rains. Numbers at Fords Bridge were especially high, with
over 10,000 mosquitoes collected from one trap in late March. These collections were
dominated by floodwater Aedes species, notably Aedes eidsvoldensis and Aedes theobaldi, and several new mosquito
species for the program were identified including Aedes
Marks no.71, Aedes Marks no.85, Aedes sapiens and Culex starckeae. Mimomyia elegans was also collected during
these floods, which is a new program record for the northwest. Along with these mosquitoes
was some late virus activity, with two isolates each of RR and SIN from Fords Bridge in late March/early April.
The isolates of RR and SIN from Aedes
eidsvoldensis and the RR from Aedes
theobaldi at Fords Bridge
represent new mosquito/virus records for NSW.
As
noted above, there were several new mosquito records from the northwest and Culex orbostiensis
was trapped for the first time from the inland at Deniliquin. An additional record
for the state was one specimen similar to Culex gelidus collected from the Sewerage
Treatment Works at Condobolin
in March 2000. This is an exotic species, first recognised in Australia from Brisbane in
June 1999 (Muller, 1999). Since then, it has been found to be widespread along coastal
Queensland (Ritchie & Haselar, 2000), throughout inland Northern Territory (Whelan et
al., 2000) and was recently discovered in Western Australia (Dr Michael Lindsay, WA
Health Department, pers. comm.). Immediately after the collection of the NSW
specimen, staff from the Entomology Department (SD & RCR) undertook an intensive
larval survey with adult trapping at Condobolin but no further specimens
were revealed. The concern is that Culex gelidus, a known vector of Japanese Encephalitis,
often breeds in polluted waters where Culex annulirostris fails to
thrive, thus if the Culex gelidus becomes established in NSW, it will
supplement the known mosquito vectors. Continual monitoring for this species is essential.
In
most years, Culex annulirostris comprises around 80%
of all mosquitoes trapped from the inland. For this season it was only around 60%, due to
the high number of Aedes trapped in the northwest, which constituted 10% of the
inland mosquitoes. However, Culex annulirostris continues
to yield the majority of inland viruses with 16 isolates (11 RR and 5 SIN). Anopheles annulipes was again
the second most common inland species, comprising roughly 20% of the collections, and 8
viruses (3 RR
and 5 unknowns) were isolated from this species.
As
stated above, there was no evidence of any MVE in NSW, despite one
of the environmental models suggesting possible activity. However, there has been widespread activity with associated
human cases during 2000 in Western Australia, the Northern Territory and in the north of
South Australia (Broom & Whelan, 2000). As evidence
suggests that MVE is introduced via
viraemic migratory birds from endemic regions of the north (Marshall, 1988), several new
trap sites were established in the northwest including Wanaaring and Fords Bridge. The expectation is
that if MVE
does come from the north into NSW, then it will be detected before reaching the more
populated areas of the south. In light of
northern MVE cases, and
that both the Forbes’ and Nicholls’ hypotheses are suggesting MVE
activity in 2000-2001, it will be critical that monitoring of weather patterns continue
into 2000. Relevant
environmental conditions appear on the NSW Arbovirus Surveillance web site.
The
number of chicken flocks was increased this year to nine, with eight surveillance
locations. Due to the MVE activity in central
Australia and the high rainfall in the northwest, testing of the flocks in the northwest
was extended through May. There were no seroconversions in the NSW flocks.
The mosquito numbers from the north coast were
mostly down on average, although there were several large collections in late March/early
April from Ballina with freshwater mosquitoes dominating. South coast numbers continued to
be very low due to the dry summer conditions, and arbovirus activity has been low since
the Barmah Forest outbreak in 1994-1995
(Doggett et al., 1999). This would suggest that both animal reservoirs and the
human population would be becoming increasingly susceptible to arboviral infection and at
risk of major activity. Continued monitoring of this region is highly advisable. New
mosquito records for the coast include Aedes
bancroftianus trapped at Port Stephens and Anopheles
amictus collected from the Hawkesbury traps.
The number of Sydney surveillance locations was also reduced
this season, with emphasis being placed on the Western Sydney locations where RR activity has occurred over the last
three years. As per elsewhere, mosquito numbers were down with the cold temperatures.
There were no viral isolates from the mosquitoes. A small number of locally acquired cases
of RR were identified in Western
Sydney, although notifications were down upon previous years.
Human
disease notifications in 1999-2000 contrasted strongly with the previous season. In 1998, the heavy rains
led to enormous mosquito numbers in the southern inland regions and the surveillance
resulted in a RR mosquito
isolate from the first week of November. Eventually, record numbers of human RR cases were reported from this region during late spring/summer. For
1999-2000, the heavy rains in the same period did not lead to large mosquito numbers,
there were no early arbovirus isolates and human cases were relatively few. The late
mosquito and arbovirus activity in the northwest occurred in a sparsely populated region,
and hence there was no significant rise in notifications. The only Area Heath Service to
have more notifications than last season was the Mid North Coast. Interestingly, this was
the region where all arbovirus surveillance activities were stopped prior to the 1999-2000
season.
